Method of producing boride coatings on metal



United States Patent METHOD OF PRODUCING BORHJE COATINGS 0N METAL Margaret A. Reid, Washington, D; C., and Abuer Brenner, Chevy Chase, MIL, assignors to the United States of America as represented by the Secretary of Commerce No Drawing. Application August 30, 1954 Serial No. 453,171

6 Claims. (Cl. 117-106) This invention is concerned with protective coatings for metals used at high temperatures, and more particularly with a method for producing boride coatings on metals -by vapor deposition of the coating onto the metal.

Better protection for metals from oxidation at high temperatures is urgently needed in many military applications, particularly in the manufacture of parts for jet aircraft and for rockets. a coating resistant to oxidation at high temperatures would enable present alloys to be used at higher temperatures and would also allow the use of certain other metals, for instance molybdenum, which has desirable mechanical properties at elevated temperatures but which has the disadvantage of oxidizing too rapidly at these temperatures.

At present ceramic coatings are used primarily in the protection of metals subjected to the high temperatures encountered in modern aircraft engines. However, these ceramic coatings have the disadvantage that they have an extremely high firing temperature-in the case of molybdenum about 1200 degrees C.

Another form of coating for molybdenum is known, a molybdenum silicide coating deposited by thermal decomposition of SiCl and H but this coating likewise requires a temperature of 1200 degrees C. for formation. Unfortunately the molybdenum base metal starts to undergo recrystallization at about 900 degrees C. and be comes very brittle. In addition, the coatings provided by this method are themselves quite brittle.

One object of this invention is to provide a protective coating for metals which can be deposited at a comparatively low temperature.

Another object of this invention is to provide a protective coating for molybdenum which can be deposited at a temperature below the recrystallization temperature of that metal.

An additional object of this invention is to provide a protective coating for metals that is relatively ductile at room temperatures.

This invention involves the production of an adherent boride coating for metals by thermal decomposition and has the advantage of the formation of such coatings at temperatures lower than the methods referred to above.

In addition the coating provided by the present inventionis one that may be electroplated, thus giving a sandwich coating that is more ductile and less brittle than the boride coating alone yet oifers suflicient protection to the base metal.

The coatings of the present invention can be produced by the decomposition of aluminum, beryllium, zinc, uranium, and other metal borohydrides.

Using conventional vapor coating apparatus, deposits have been obtained in accordance with the principles of this invention from the thermal decomposition of aluminum borohydride and also beryllium borohydride, which have afforded molybdenum protection in an oxygengas flame. The aluminum boride deposits were established The protection offered by- 2,849,336 Patented Aug. 26,, 19.58

in a decomposition furnace at atemperature ofapproximately 700- degrees C. and. a pressure. of 5 to. 10 millimeters of'mercury. Whilethe pressure is notcriticalit shouldbe; maintained at such. a value that the borohydride concentration does not increase to a value atwlii'chcom: plete decomposition takes place. By Way of. exampl'e fit has been found that 2.0 ml. of borohydride provide a. deposit of 014 g. of aluminumrboridfe. p Y

The aluminum boride deposit has the approximate formula, AlB and thus differs from the product obtained by direct combination of aluminum and boron which has the formula Al B The aluminum boride deposits vary in color from a white metallic appearance to almost black. The black deposits are smooth but can be scraped with a knife edge. The white deposits are very hard and can scarcely be scratched with a file. The boride is chemically resistant, dissolving very slowly in hot nitric acid. The aluminum boride coating is conductive and may be electroplated with other metals.

In each case the temperature of decomposition varies with the nature of the substance used, but in general is above 400 degrees C. It has been determined that in most cases the optimum temperature range should be from 600-800 degrees C.

The boride coatings formed in accordance with this invention also have numerous practical uses such as, f r example, the protection of metals against oxidation, chemical attack and abrasion.

Two very important variables to be considered in the production of these coatings are the flow rate of the gas vapor and the distribution of the flow. A too rapid flow will give a rough, treed deposit, and an uneven flow will result in an uneven deposit. The borohydride gases must be distributed equally over the entire surface to be coated. It is desirable that evacuation of the waste gases be provided so that the pressure inside the decomposition chamber is never more than a few millimeters of mercury. A small amount of inert gas or hydrogen may be used as a carrier.

While the adherence of the coating to all metals described has been satisfactory, the aluminum boride coating best adheres to the following metals in the order listed: nickel, chromium, stainless steel, and molybdenum. Dilferent surface treatments of the various metals do not appear to be a factor in the adherence qualities. In the boride coatings listed above, the coating did not adhere to the base metal as Well when the metal had been treated in purified hydrogen at red heat. Apparently an oxide film tends to improve the adherent properties of the coating.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made Within the scope of invention as defined in the appended claims.

What is claimed is:

1. The method of coating a metal comprising passing a metallic borohydride vapor over the surface of the metal to be coated at a temperature between 400 and 800 degrees C. whereby metallic boride from said vapor is deposited as a coating on said metal.

2. The method of coating a metal with a boride comprising subjecting the metal to a mixture of gases containing a metallic borohydride vapor at a temperature between 400 and 800 degrees C. whereby metallic boride from said vapor is deposited as a coating on said metal.

3. The method of coating a metal comprising subjecting the metal to a mixture of gases containing a metallic borohydride vapor at a temperature between 400 and 800 degrees C. and maintaining said mixture at pressure of a few millimeters of mercury whereby metallic boride from said vapor is deposited as a coating on said metal.

. 3 r 4 4. Themethod of coating a metal comprising sub- 6. The method of coating a metal comprising equally jecting the metal to be coated to a gas vapor mixture indistributing a metallic borohydride gas vapor over the eluding a volatile metallic borohydride vapor and an entire surface of the metal to be coated at a temperature inert gas at atemperature of between 400 and 800 debetween 400 and 800 degrees C. whereby metallic boride grees C. and maintaining this gas mixture at a pressure of 5 from said vapor is deposited as a coating on said metal.

'a few millimeters of mercury whereby metallic boride from said vapor is deposited as a coating on said metal.

5. The method of coating 2. metal comprising sub- Hoekstra et al.; I. Am. Chem. Soc., 71, pp. 2488 to 'jecting the metal to be coated to an aluminum borohy- 2592, July 1949. (Copy in Science Library.)

dride vapor at a temperature between 400 and 800 de- 10 Campbell et al.; I. Electrochemical Soc., 96, 318-331,

grees C. whereby aluminum boride from said vapor is November 1949. (Copy in Science Library.)

deposited as a coating on said metal.

References Cited in the file of this patent 

1. THE METHOD COATING A METAL COMPRISING PASSING A METALLIC BOROHYDRIDE VAPOR OVER THE SURFACE OF THE METAL TO BE COATED AT A TEMPERATURE BETWEEN 400 AND 800 DEGREES C. WHEREBY METALLIC BORIDE FROM SAID VAPOR IS DEPOSITED AS A COATING ON SAID METAL. 